$(a)$ State two main causes of a person developing near-sightedness (myopia). With the help of a ray diagram, suggest how this disability can be corrected.
$(b)$ The far point of a myopic person is $150 \ cm$ in front of the eye. Calculate the focal length and power of the lens required to enable him to see distant objects clearly.

(N/A) The two main causes of myopia are:
$1$. Excessive curvature of the eye lens.
$2$. Elongation of the eyeball.
To correct this, a concave lens of suitable focal length is used. The lens diverges the incoming parallel rays from a distant object so that they appear to come from the far point of the myopic eye, allowing the image to be formed on the retina.
$(b)$ Given:
Far point of the myopic eye $(v)$ = $-150 \ cm = -1.5 \ m$.
For a distant object, the object distance $(u)$ = $\infty$.
Using the lens formula: $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$.
$\frac{1}{f} = \frac{1}{-150} - \frac{1}{\infty} = \frac{1}{-150} - 0 = -\frac{1}{150}$.
Thus, the focal length $(f)$ = $-150 \ cm = -1.5 \ m$.
Power $(P)$ = $\frac{1}{f(in \ meters)} = \frac{1}{-1.5} = -0.667 \ D \approx -0.67 \ D$.